The present invention relates to an end seal for nuclear instrumentation cable in which coaxial cable is hermetically sealed and terminated by the end seal. There is a need for instrumentation cables which can operate in the high temperatures and high voltages conditions of a liquid metal fast breeder reactor for in-vessel sensors. At the present time test setups have had to use a cooled thimble design to keep the cable at a relatively low temperature.
The key difficulty in developing instrumentation cable assemblies capable of operation at elevated temperature and voltage is the problem of breakdown pulse noise. Breakdown pulse noise is caused by small electrical discharges, with a charge content of approximately 10-13 coulombs, which occur across ceramic insulators at elevated temperature when high dc voltages are applied. This electrical noise prevents the acceptable operation of the nuclear instrumentation sensor, which can be a radiation flux detector. The component most susceptible to breakdown pulse noise has been the coaxial cable end seal.
The instrumentation cable is a coaxial design with a conductive tubular sheath, a metal oxide insulator within the tubular sheath, and a conductive center wire. The cable is typically subjected to a high pressure of an insulating gas such as nitrogen when the cable is hermetically sealed at the end seal. The hermetic end seal protects the interior of the cable from moisture and other contamination, and maintains the fill gas within the cable, which is especially critical for reliable operation of fission chamber detectors to which the cable is connected.
The end seals used to date are of a coaxial design in which a ceramic insulator body is disposed within and braze sealed to a conductive tubular sheath which is braze sealed or welded to the cable sheath. A center conductor also passes through and is sealed to the ceramic insulator body, which center conductor is connected to the cable center wire. There is a gas filled chamber defined between the end of the cable insulator and the end seal ceramic insulator body. The ceramic body has metallized surface portions to permit the braze sealing.
The temperature and voltage ratings for such end seals have been improved by providing steps or convolutions in the ceramic face, and by controlling the angle of the ceramic face to effect the breakdown pulse noise characteristics.
It has been discovered by the present inventors that breakdown pulse noise originates primarily at the end seal surface where the ceramic face, the conductive sheath or center wire, and the fill gas atmosphere are present at a "triple point" junction. Since the center wire carries the high voltage signal the problem is more serious at the center wire, ceramic face, fill gas, "triple point" junction. The braze seal which is made at this "triple point" junction sealing the metallized ceramic surface to the center wire or the outer tubular sheath, is subjected to a high voltage radial field, which apparently causes some electron emission and secondary emission along the face of the ceramic extending in the radial direction.